Of all the logistical challenges to fix when a flight goes tech, likely handling the catering issues is the least significant for a not merely short, but micro-haul. The BOB alternative is perfectly fine on a flight such as this.
The simple answer here is that the crew forgot to tell OP that he could have something off the BOB menu and thus he did not. That is a service failure which BA can easily remedy with a few avios. No need to turn this into a massive contractual dispute.
Better training also helps at the gate. When OP was told on the ground that there was no meal, it should have been accompanied with a, "but you may have something off the BOB menu as well," thus allowing OP to decline the rebooking and wait for the next available flight with a meal if that was his primary concern.